We will complete a case-control study of 300 homosexual men with gastrointestinal symptoms and 100 asymptomatic homosexual men aimed at correlating the clinical syndromes of proctitis, proctocolitis, and enteritis with specific infectious agents. All patients will undergo comprehensive microbioligic and serologic studies to evaluate the role of potential causative agents. Fiberoptic sigmoidoscopy will be utilized to define the extent of disease and to obtain rectal biopsies in all symptomatic men and in one-third of asymptomatic men. The efficacy and costs of empiric vs specific treatment regimens for the management of gastrointestinal disease syndromes in homosexual men will be compared in a therapeutic trial. Our studies will be integrated with an ongoing study of risk factors associated with subclinical immunosuppression, idiopathic lymphadenopathy, and AIDS in homosexual men. Thus, we will determine the relationship between these important entities and the prevalence, type, chronicity, and clinical expression of gastrointestinal infection in homosexual men. In related laboratory studies, we will further characterize the novel campylobacterlike organisms (CLO) we have isolated from homosexual men with proctocolitis, particularly their genetic relationship to known compylobacter species, the feasibility of diagnosing these infections by a DNA probe technique, experimental infection with CLOs in pigtailed mecaques, and evaluation of the IgM and IgG antibody response to CLO infections in humans. We will also characterize the histopathology of gastrointestinal infection with STD pathogens utilizaing monoclonal antibodies to C. trachomatis, N. gonorrhoeae, T. pallidum, Herpes Simplex virus, Ureaplasma Urealyticum, and Mycoplasma hominis, and to T & B lymphocytes.